Ding, dong, dell, Ken's in the well. Actually, it's more of a canal than a well, and it's Eccles the dog rather than his blow-dried master who finds himself thrashing for breath, but the outcome of the splosh-based scenario is the same. To wit: HELP. Enter Martha (Stephanie Beacham), with a butterfly net, a barge and an enormous bob, thus promising much in the way of canine/romantic salvation ("I've a rather meaty shiraz . . . "). Come, join the love boat.

Horizon9pm, BBC2

Ten bony volunteers spend a month scoffing 5,000 calories a day in an effort to address the dieter's most vexed bugbear: how come thin people aren't fat? The answer - a bewildering blend of genes, metabolism and conditioning - emerges from between the adipose flaps of a documentary that turns a familiar experiment (Supersize We?) into an unexpectedly chipper study of biological injustice. While moats of custard and beef mesas are forced down increasingly queasy craws, experts put magnifying glasses to the latest obesity theories. A family bucket of science, then: with fries.

Unforgiven9pm, ITV1

Sally Wainwright's superlative drama concludes in a hail of realisations and revelations, as Ruth (Suranne Jones) tracks down estranged sister Katie/Lucy (Emily Beecham), fatally misguided Steve (Matthew McNulty) sharpens his resolve, and the past empties its handbag over the kitchen floor. "People do what they have to do," shrugs Ruth, as guilt grinds its heel ever deeper into a fragile future.

What Darwin Didn't Know9pm, BBC4

Biologist Armand Leroi launches Auntie's Darwin season with a suitably celebratory examination of the evolution of evolutionary theory. From the unexpected roots of Darwin's hypotheses (France? Mais oui) to the embittered Professor Calculus types who picked up his notional baton and proceeded to bash one another about the moustache with it, it's a study made all the more crackly by Leroi's fossil-dry asides. Among the case studies are an Edwardian brainiac who crossbred 53,000 evening primroses. "Well," drawls Leroi, eyebrows hovering like bitchy zeppelins, "he was Dutch ... "